1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to satellite digital audio service (SDARS) tuner architectures.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Satellite radio operators will soon provide digital quality radio broadcast services covering the entire continental United States. These services intend to offer approximately 100 channels, of which nearly 50 channels will provide music with the remaining stations offering news, sports, talk and data channels. According to C. E. Unterberg, Towbin, satellite radio has the capability to revolutionize the radio industry, in the same manner that cable and satellite television revolutionized the television industry.
Satellite radio has the ability to improve terrestrial radio""s potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are owned by CD Radio and 12.5 MHz of which are owned by the assignee of the present application xe2x80x9cXM Satellite Radio Inc.xe2x80x9d. The FCC further mandated the development of interoperable receivers for satellite radio reception, i.e. receivers capable of processing signals from either CD Radio or XM Radio broadcasts. The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters will broadcast the same program content in order to improve coverage reliability. Some mobile receivers will be capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation against multipath and blockage of the satellite signals. In accordance with XM Radio""s unique scheme, the 12.5 MHz band will be split into 6 slots. Four slots will be used for satellite transmission. The remaining two slots will be used for terrestrial re-enforcement.
In accordance with the XM frequency plan, each of two geostationary Hughes 702 satellites will transmit identical or at least similar program content. The signals transmitted with QPSK modulation from each satellite (hereinafter satellite 1 and satellite 2) will be time interleaved to lower the short-term time correlation and to maximize the robustness of the signal. For reliable reception, the LOS signals transmitted from satellite 1 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by non line-of-sight (NLOS) terrestrial repeaters. The assigned 12.5 MHz bandwidth (hereinafter the xe2x80x9cXMxe2x80x9d band) is partitioned into two equal ensembles or program groups A and B. The use of two ensembles allows 4096 Mbits/s of total user data to be distributed across the available bandwidth. Each ensemble will be transmitted by each satellite on a separate radio frequency (RF) carrier. Each RF carrier supports up to 50 channels of music or data in Time Division Multiplex (TDM) format. With terrestrial repeaters transmitting an A and a B signal, six total slots are provided, each slot being centered at a different RF carrier frequency. The use of two ensembles also allows for the implementation of a novel frequency plan which affords improved isolation between the satellite signals and the terrestrial signal when the receiver is located near the terrestrial repeater.
In any event, there is a need for a receiver capable of receiving both ensembles transmitted by satellite and transmitted by terrestrial repeaters. The conventional solution would require two radio frequency (RF) tuner front ends, one for satellite signals and the other for terrestrial signals. Unfortunately, the use of dual RF tuners adds significantly to the cost of the receiver inasmuch as each would require an antenna, a mixer/down-converter, at least one low noise amplifier, filters and etc.
Accordingly, a need exists in the art for inexpensive satellite digital audio radio receiver architecture capable of receiving both ensembles transmitted by satellites and terrestrial repeaters.
The need in the art is addressed by the satellite digital audio radio receiver and method of the present invention. The inventive receiver includes a circuit for down converting a first ensemble in a received combined signal in a first mode of operation and for down-converting a second ensemble from the received combined signal in a second mode of operation. The first ensemble includes a first signal received from a first transmitter, a first signal received from a second transmitter, and a first signal received from a third transmitter. The second ensemble includes a second signal received from the first transmitter, a second signal received from the second transmitter, and a second signal received from the third transmitter. The first ensemble comprises first, second and third frequency slots and the second ensemble comprises fourth, fifth, and sixth frequency slots. A controller is included to selectively switch the circuit from the first mode to the second mode.
In the illustrative embodiment, the first and second transmitters are mounted on first and second satellites and the third transmitter is a terrestrial repeater. Both ensembles are transmitted in accordance with the XM frequency plan. The first ensemble is down-converted using low side injection and the second ensemble down-converted using high side injection.
The inventive circuit includes a switchable dual voltage controlled oscillator. The circuit further includes a first intermediate frequency down-conversion stage with a first mixer for mixing the received combined signals with the output of the voltage-controlled oscillator. The circuit further includes first and second filters for separating the first and second signals received from the third signals. In the illustrative embodiment, the first and second filters are surface acoustic wave filters.
The inventive circuit further includes a second intermediate frequency down-conversion stage having second and third mixers for mixing the outputs of the first and second filters, respectively, with the output of a local oscillator. In the illustrative embodiment, the frequencies are chosen so that the second mixer and the third mixer receive a reference input from a single local oscillator.